The present invention relates to radiation and/or particle detectors, cameras and imagers, e.g. position sensitive proportional gas counters, scintillation counters, radiation detectors, polarimeters, cameras and imagers, in particular X-ray detectors, X-ray polarimeters, X-ray cameras and X-ray imagers as well as methods of manufacturing the same. The words sensor and detector are considered synonymous in accordance with the present invention.
There have been many known attempts to produce a digital medical X-ray imager. The energy distribution of the X-rays in the image plane is determined by the irradiated subject matter, human tissue and bone, resulting in an almost continuous energy spectrum both spatially and on an intensity scale. The image is generally a low contrast image whose gray tones need to be preserved for accurate diagnosis. Such an X-ray image has conventionally been recorded successfully on film in an analogue way. Digital recording devices determine the intensity averaged over picture elements or pixels. The physical detection process and the spacing of the pixels determines the resolution of the device. The intensity at each pixel is normally binned into one of a finite number of levels. Hence, the gray scale resolution is limited to this number of levels rather than being a continuous spectrum of values. All known devices attempt to optimize resolution and gray scale performance but are usually limited in one aspect or another.
The X-rays may be detected directly or indirectly, e.g. a direct conversion of X-rays to an electrical output or first a conversion to visible light and then recording the visible light pattern as shown schematically in FIG. 1.
Direct conversion devices include the use of an layer of selenium for attenuating the X-rays and generating free electron-hole pairs for collection by suitable electrodes. They also include other methods such as conversion of X-rays in solid or gas media followed by multiplication of emitted electrons, conversion and charge collection in pixelised CCD cameras.
Indirect conversion devices use phosphors for converting the X-rays into visible light. One improvement has been photostimulable phosphors, known as storage phosphors. These store the image for later activation by red light. The phosphor can be xe2x80x9creadxe2x80x9d by a scanning laser and emits blue light in accordance with the stored image. The emitted light is collected and detected by a photomultiplier.
Another improvement is represented by the XRII described in the article by P. M. DeGroot, xe2x80x9cImage intensifier design and specificationsxe2x80x9d, Proc. Summer School on Specification, Acceptance, Testing and Quality Control of Diagnostic X-ray Imaging Equipment, Woodbury, USA, 1994, pages 429-60. In this device X-rays are converted to light in a large curved phosphor screen. The resulting fluorescence illuminates a photocathode which liberates electrons and is evaporated directly onto the inside of the phosphor. The electrons are accelerated through a large electric potential, e.g. 25 kV, and electrostatically focussed by electrodes onto a small diameter output phosphor which may be observed by a video camera. The device has the advantage that the electron energy is increased by the acceleration thus counteracting to a certain extent some of the conversion loss but has the serious disadvantages of being very bulky giving limited access to the patient, image distortion, loss of image contrast and high cost and complexity.
A microgap sensor is known from the article xe2x80x9cThe Micro-gap Chamberxe2x80x9d, by F. Angellini et al., Nuclear Instruments and Methods in Physics and Research, Sect. A (1993), pages 69-77. Such devices can detect X-rays however they are mainly used for detection of particles. Microdot detectors as described by Biagi et. al. in the article xe2x80x9cFurther experimental results of gas microdot detectorsxe2x80x9d, Nuclear Instruments and Methods in Physics Research, A392 (1997) pp 131-134, 4th International Conference on Position Sensitive Detectors, Manchester, Sep. 9-13, 1996 use an electrode arrangement as shown schematically in FIGS. 2A and B. FIG. 2A shows a top view of the electrodes which include anodes 6 in the center connected to an anode readout bus 4 and cathodes 7 arranged around each anode 6 and connected to a cathode readout bus. Between the cathode 7 and the anode 6 one or more field electrodes 5 may be placed. The planar electrode arrangement and read-out busses may be conveniently made using standard semiconductor processing techniques starting from a planar substrate such as a glass plate or a semiconductor, e.g. silicon wafer 9, oxidizing the wafer to form an insulating layer 8 and depositing the electrodes 5,6,7. A gap 3 (typically 3 mm) is provided between the planar electrode arrangement and a cathodic drift electrode 2 which is filled by an ionisable gas. An electric potential is provided between the drift electrode and the electrode arrangement. A high energy charged particle entering the device via the drift electrode 2 causes release of at least one electron. This electron is accelerated towards the nearest anode or anodes 6 causing further collisions with atoms of the ionisable gas thus releasing more electrons until an avalanche is produced. The arrival of the avalanche at one or more anodes 6 is detected by electronic circuitry connected to the read-out busses. These known devices can detect high-energy charged particles and record a complex 2-D X-ray image but at low detection efficiency. Hence, they are unsuitable for detecting complex X-ray images.
An object of the present invention is to provide a particle or radiation detector or a radiation, especially an X-ray imager which provides good localization resolution and a wide intensity scale.
A further object of the present invention is to provide a particle or radiation detector or a radiation, especially an X-ray imager with reduced bulk.
Another object of the present invention is to provide a particle or radiation detector or a radiation, especially an X-ray imager with reduced manufacturing cost and complexity.
Yet another object of the present invention is to provide a large size particle detector or a radiation, especially an X-ray imager.
The present invention provides a microgap detector comprising: a planar cathode; at least an anodic electrode on a first insulation layer, said anodic electrode being separated and insulated from said cathode by said first insulation layer; a planar cathodic drift electrode substantially parallel to said planar cathode, said planar cathode and said drift electrode being separated by a gap fillable with an ionisable gas, said gap being considerably greater than the thickness of said first insulating layer, said anodic electrode being located in the gap between said cathode and said drift electrode, and said anodic electrode being connected to an underlying conductive layer through a via hole in said first insulating layer. Preferably an electronic measuring device is provided with each anodic electrode, preferably immediately below and within the area thereof. The measuring device may be a charge storing device such as a capacitor or a digital counter, for instance.
The present invention also provides a method of manufacture of a microgap detector, comprising the steps of: forming a substrate with a first conductive layer, forming a second conductive planar cathode layer, depositing a first insulating layer onto said second conductive planar cathode layer, forming a via hole through said first insulating layer to said first conductive layer, forming at least one anodic electrode on said first insulating layer in contact with said first conductive layer through said via hole, mounting a cathodic drift electrode substantially parallel to said first conductive cathode layer, said first conductive cathode layer and said cathodic drift electrode being separated by a gap, said gap being considerably greater than the thickness of said first insulating layer and said anodic electrode lying between said first conductive planar cathode layer and said cathodic drift electrode.
The method of manufacture is particularly suitable for the production of gaseous electron multipliers for large size X-ray imagers independent of limitations imposed by silicon wafer processing equipment.
The present invention also includes a detector panel comprising: a planar cathode, a two dimensional array of a plurality of anodic pixel electrodes on a first insulation layer, each said anodic pixel electrode being separated and insulated from said cathode by said first insulation layer, a planar cathodic drift electrode substantially parallel to said planar cathode, said planar cathode and said drift electrode being separated by a gap fillable with an ionisable gas, said gap being considerably greater than the thickness of said first insulating layer, said anodic pixel electrodes being located in the gap between said cathode and said drift electrode, and a plurality of electronic measuring elements, each electronic measuring element being connected to one of said anodic pixel electrodes. The electronic measuring elements may be one of a charge storing device, e.g. a capacitor and a digital counter. The detector panels may be placed adjacent to each other to form a strip imager or be placed in a two-dimensional overlapping array to form a rectangular imager.
The present invention with its advantages and embodiments will be described with reference to the following drawings. The dependent claims define further individual embodiments of the present invention.